The published literature on contact lenses has been extensive since the disclosure of the use of organic plastics for this purpose by Toughy in U.S. Pat. No. 2,510,438. The standard contact lens used almost exclusively for many years was made from poly(methylmethacrylate) (PMMA). PMMA can be readily injection molded, compression molded, or cast molded to give inexpensive, highly transparent, stable, hard, wettable contact lenses. However, as a result of the very low oxygen permeability of PMMA lenses made therefrom have to be removed from the eye at least once daily.
In order to overcome the deficiency of the PMMA lenses, numerous other organic polymers have been proposed for use as contact lenses. Cellulose acetate butyrate lenses, for example, have a somewhat higher oxygen permeability than PMMA and are readily fabricated by injection molding but have a lower dimensional stability than PMMA.
Soft contact lenses based upon crosslinked polymers from hydrophilic monomers, such as 2-hydroxyethyl methacrylate or N-vinyl-2pyrrolidone, are found to be generally more comfortable to wear than PMMA lenses but are more fragile and have a tendency to become cloudy because of the ease by which proteinaceous material and other contaminants are deposited thereon. Soft lenses prepared from silicone rubber have a very high oxygen permeability but poor mechanical properties and are resistant to wetting.
Compared with PMMA contact lenses, lenses prepared from copolymers of a silicone methacrylate with methyl methacrylate (Gaylord; U.S. Pat. No. 4,120,570) or with methyl itaconate (Ellis; U.S. Pat. No. 4,424,328) have significantly greater but still limited oxygen permeability. Considerably higher oxygen preferabilities for contact lenses from crosslinked methyl methacrylate, fluoro substituted methacrylates and siloxane methacrylate copolymers have been reported by Ratkowski and Burke (U.S. Pat. No. 4,661,573). Conventional lathe cutting techniques are used to convert the silicone methacrylate type copolymers to hard contact lenses. An extensive summary of related contact lens art is given by Keogh (U.S. Pat. No. 4,259,467).
Although the crosslinked, highly oxygen permeable hard contact lenses now available represent a considerable advance over the prior art, the fabrication of these lenses requires an expensive, delicate and time consuming lathing operation in order to obtain good optical properties.
Wichterle (U.S. Pat. No. 3,408,429) describes a complex centrifugal casting technique for converting a mixture of hydroxyethyl methacrylate and ethylene glycol dimethacrylate into a crosslinked polymer lens which, upon being subjected to normal saline, is converted into a soft contact lens. This complex procedure inherently yields products having optics that are inferior to those of hard contact lenses.
Considerable attention has been given to the modification of polymer properties through the use of procedures involving the formation of an interpenetrating polymer network (IPN). An IPN can be defined broadly as an intimate network of two or more polymers at least one of which is either synthesized and/or crosslinked in the presence of the other(s).
Liu, for example, in U.S. Pat. No. 4,618,644 describes the polymerization of methyl methacrylate in the presence of a silicone polymer to obtain a product of improved toughness. The polymerization of hydroxyethyl methacrylate in the presence of ethylene glycol dimethacrylate and a crosslinkable poly(dimethylsiloxane) to yield a product stated to be useful for the fabrication of contact lenses is described by Falcetta (Ger. Offen. D.E.2,518,904). Contact lenses have also been fabricated from the interpenetrating network polymer resulting from the polymerization of 2-hydroxyethyl methacrylate in the presence of poly-N-vinylpyrrolidone (Ewell, U.S. Pat. No. 3,647,736).
Neefe (U.S. Pat. No. 4,632,773 ) shows the polymerization of methyl methacrylate in the presence of a syrup containing polymerized methacryloxypropyltrimethoxysilane and a fluorescent colored pigment to obtain a solid contact lens blank material which can be readily identified. Tighe and Gee (U.S. Pat. No. 4,430,458) disclose the formation of a soft contact lens material by the crosslinking of a polymeric hydrogel of a copolymer of N-vinyl-2-pyrrolidone during the final compression or injection molding process. Lim et al (U.S. Pat. No. 4,536,554) describes the preparation of soft contact lenses made from the interpenetrating network polymer obtained by the polymerization of a mixture containing a hydrophilic and a hydrophobic monomer and at least two crosslinking agents. A comprehensive review of interpenetrating polymer networks and their applications is provided in the book on Interpenetrating Polymer Networks and Related Materials by L. H. Sperling: Plenum Press; New York and London; 1981.
As noted above, considerable progress has been made in polymer synthesis and fabrication since PMMA contact lenses were first introduced. The need, however, for a highly oxygen permeable, durable, dimensionally stable, comfortable contact lens which can be readily fabricated by a simple economical procedure is still an unfilled need of the optical industry and it is toward this need that the present invention is directed.